1. Field of the Invention
The present invention pertains to the art of high frequency coaxial transmission lines, and more particularly to the art of forming glass-to-metal seals in high frequency coaxial transmission lines.
2. Art Background
Transmission lines for high frequency signal propagation typically consist of two conductors separated by a dielectric material that can hold an electric charge. The two important characteristics of a transmission line are its impedance and maximum operating frequency, both of which are determined by the relative size and spacing of the conductors and the dielectric constant of the material separating them. Maximum operating frequency is limited by the fact that if the dimensions of the transmission line are greater than a certain fraction of the wavelength that is being propagated, then unwanted modes develop which are detrimental. Therefore, as the operating frequency of the transmission line increases, the characteristic dimensions of the transmission line components must be decreased. Control of line impedance is critical since a fraction of the signal is reflected whenever there is an impedance mismatch. As a result, it is necessary to maintain constant impedance through the entire signal path in order to minimize the amount of unwanted reflections that occur when there is a mismatch.
Air is a common dielectric used in high frequency coaxial transmission lines. Such transmission lines require supports to maintain the coaxial placement of the center conductor, and also require supports at connectors. In such supports, a different dielectric material, such as a fluorinated polymer, ceramic, glass, or glass-ceramic material is used. As such materials represent a change in dielectric constant from air, the geometry of the transmission line must be altered to maintain as close to a constant impedance as possible. Since the dielectric constant of the material used to support the center conductor is typically higher than that of air, the diameter of the inner conductor must be reduced, or the diameter of the outer conductor increased, in order to maintain pro per impedance. At higher frequencies, the line is more susceptible to discontinuities and the geometry of the air to glass transition must be closely controlled.
The placement of the support with respect to the change in diameter of the center conductor is also critical. The geometry of this transition region must be carefully controlled to maintain the required characteristic impedance through the tapered transition region, and to minimize reflections. Particularly at millimeter-wave frequencies, small shifts or distortions in the relative position of the support to the tapered center conductor can result in changes in return loss on the order of 30 dB or more. Therefore, tolerances in the support must be carefully controlled.
The seal between the support and conductors in a typical glass to metal seal results from either chemical bonds that form between the glass and metal, depending on the composition of the metal, or compressive stresses that develop in the glass during processing. Compressive stresses develop when the coefficient of thermal expansion of the metal exceeds that of the glass, and are desirable in that it increases the ruggedness of the structure. Glass is very weak in tension, which develops when the center conductor is flexed radially. If the glass is pre-stressed compressively, these forces must be overcome before the tensile strength of the glass becomes a concern. As the dimensions of the transmission line decrease, however, it is more difficult to achieve the pre-stressing that is necessary for good reliability in the field.
A support structure for high frequency coaxial transmission lines uses a split conductive bead having locking provisions and relief areas to contain excess glass from the dielectric glass bead sections surrounding the tapered center conductor. A fixture for forming the glass to metal seal controls the positioning of the glass with respect to the taper in the center conductor.